The polyethylene terephthalate films are excellent in physical and chemical properties, especially in flatness, mechanical strength and dimensional stability, if compares with other films, and therefore, they have been widely used as base films for magnetic recording media.
Recently, the miniaturization and the increase in recording density of the magnetic recording media including video tape has been rapidly made. Because the high-density magnetic recording is effective at short wavelength, the thickness of a magnetic layer in the high-density magnetic recording medium is reduced so as to improve an output loss due to the thicker magnetic layer. For example, a standard video tape has generally a magnetic layer of about 5 .mu.m, while a high-density video tape where a metal powder, mainly a pure iron powder is applied has the magnetic layer of about 2 .mu.m. With the reduction in thickness of the magnetic layer, the influence of surface properties of the base film on the electromagnetic properties of the magnetic recording medium becomes more larger. Therefore, the surface of the base film should be as flat as possible.
Until now, the improvement in flatness of the base film is usually accompanied by the damages of other properties such as slipperiness, abrasion resistance, scratch resistance and protuberance resistance were impaired. Using a base film having poor abrasion resistance, a white powder produced due to the friction and abrasion between the film and various rollers during preparing the tape. It inhibited the uniform application of the magnetic layer and was partly transferred on the surface of the magnetic layer, which caused a drop-out. The white powder also produced during a calendering step after applying the magnetic layer to the film so as to be made into the magnetic tape. Then, it deposited on a surface of a calendering roll, which gave the rough surface of the magnetic layer and caused the lowering in electromagnetic properties of the resultant magnetic tape.
It is known to incorporate inactive fine particles in the film so as to inhibit the production of the white powder and to improve the abrasion resistance. By incorporating such particles in an amount enough to obtain a satisfactory abrasion resistance, naturally the flatness of the film is damaged.
The protuberance resistance is one of the properties which are requested for the base film. The term "protuberance" means herein a phenomenon which, when the base film is wound under tension, causes a partly protuberance due to the inclusion of a dust or other contaminants as well as the carrying of the concomitant air produced by winding at high speed. When the base film protuberated, it deformed in the form of circular projection, which also caused the drop-out.
It is known to incorporate particles effective for the improvement of the slipperiness in the base film so as to inhibit the protuberance. By incorporating such particles in an amount enough to obtain a satisfactory protuberance resistance, naturally the flatness of the film is damaged.
With the miniaturization of a magnetic recording system, the reduction in thickness of the tape, i.e. the base film is also made. Because the base film having the reduced thickness is poor in mechanical strength, the film is generally subjected by stretching in machine direction or in both machine and transverse directions at high draw ratio so as to increase the mechanical strength. Such a film is generally called "a tensilized film". However, the tensilized film is easily scratched. Especially if has the flat surface, the production of scratch increases, which invites the lowering in output and causes the drop-out, when made into the magnetic tape. Therefore, the flat film which is hardly scratched could not be obtained. In addition, the tensilized film has the poor abrasion resistance. By stretching at high draw ratio, large void at the interfaces between the particles incorporated in the film and the film are formed, thereby the particles are easily dropped out and the white powder is easily produced during the calendering step.
An object of the present invention is to provide a base film for magnetic recording medium, which has a flat surface and is excellent in abrasion resistance, scratch resistance and protuberance resistance when made into the tensilized film.